An arrhythmia is an abnormal heart beat pattern. One example of arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Another example is a tachycardia wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heart beat abnormally fast. With ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a tachycardia is typically not fatal. However, atrial tachycardia can trigger atrial fibrillation (AF) wherein the atria of the heart beat chaotically reducing the efficiency by which blood is pumped from the heart. Ventricular tachycardia can trigger ventricular fibrillation (VF) wherein the ventricles of the heart beat chaotically resulting in little or no net flow of blood from the heart to the brain and other organs. If not terminated, VF is fatal. Death resulting from VF represents one type of sudden cardiac death (SCD). Hence, it is highly desirable to prevent or terminate arrhythmias, particularly arrhythmias of the type that may lead to VF.
A variety of implantable cardiac stimulation devices have been developed to monitor the heart to detect arrhythmias and to administer appropriate therapy. Pacemakers are implantable devices programmed to recognize certain arrhythmias such as bradycardia or tachycardia and to deliver low-voltage electrical pacing pulses to the heart using various pacing leads implanted within the heart in an effort to remedy the arrhythmia. For bradycardia, for example, the pacemaker may be programmed to pace the heart whenever the natural or “intrinsic” heart rate falls below a programmed base rate, thereby preventing abnormally slow heart rates. A determination of when to deliver individual pacing pulses is typically made using various routine programmable parameters such as AV/PV delay and refractory period.
Some pacemakers are also programmed to overdrive pace the heart above the base rate in an attempt to prevent a tachycardia from occurring, and thereby help prevent AF or VF from being triggered. Briefly, with overdrive pacing, the cardiac stimulation device paces the heart so that most beats are paced beats rather than intrinsic beats. To this end, the device occasionally determines the intrinsic heart rate of the patient from a pair of intrinsic beats and then paces the heart at a rate typically five or ten beats per minute (bpm) faster than the intrinsic rate. Thus, if the intrinsic rate exceeds the base rate, the heart will be paced at a still higher overdrive rate. If the intrinsic rate falls below the base rate, the heart usually will be paced at the base rate. In either case, most resulting beats are paced beats rather than intrinsic beats. Typically, the base rate and overdrive rate are programmed by the physician so as to achieve a high degree of overdrive pacing, as represented by the percentage of paced beats out of total beats. In many patients, overdrive pacing helps prevent a tachycardia from occurring and, if a tachycardia nevertheless occurs, overdrive pacing at a still higher rate than the tachycardia can help terminate the tachycardia and reduce the risk that the tachycardia might trigger a fibrillation, either AF or VF.
Pacemakers, however, are usually not capable of terminating AF or VF if such as fibrillation nevertheless occurs. High-voltage electrical shocks typically must be delivered to the heart to terminate fibrillation. Hence, patients prone to AF or VF are usually provided with an ICD, which is an implantable cardiac stimulation device capable of delivering the necessary high-voltage electrical shocks to the heart when AF or VF occurs. The ICD includes a set of defibrillation capacitors for storing charge. In use, when it appears that a defibrillation pulse may need to be delivered, the ICD charges the capacitors to high voltage levels and then, if a pulse is indeed necessary, the ICD delivers pulse to the heart of the patient. ICDs may also be configured to perform routine pacing functions, such as base rate pacing or overdrive pacing. Both pacemakers and ICDs typically gather and record a substantial amount of diagnostic data pertaining to the patient and to the device itself. Diagnostic data pertaining to the patient may include internal electrocardiograms (IEGMs) and the detection of such events as premature atrial contractions (PACs) or premature ventricular contractions (PVCs). Diagnostic data pertaining to the patient may include the impedance of the leads used for pacing and the voltage of the power supply of the device.
Some state-of-the-art implantable cardiac stimulation devices are programmed to detect when the patient is in a state of profound rest (such as sleep) and to reduce the base pacing rate while the patient is at rest. This often makes it easier for the patient to sleep and also conserves battery resources within the pacemaker. To this end, the pacemaker may be provided with an activity sensor, which detects the amount of physical activity of the patient. If the activity level is very low for a predetermined period of time, a determination is thereby made that the patient is at rest and the device switches to a rest mode wherein the base rate is lowered.
Although the detection of whether the patient is at rest has been advantageously used to lower the pacing base rate, it does not appear that any state-of-the-art cardiac stimulation devices have used rest detection for use in adjusting other functions of the device. It may be beneficial to also adjust other pacing parameters such as overdrive pacing parameters to, for example, lower overdrive pacing rates while the patient is at rest make it even easier for the patient to sleep and to further conserve battery power. It may also be beneficial to adjust diagnostic-gathering parameters of the device based on whether the patient is at rest, such as the specific types of data to be gathered or how frequently the data is to be gathered, also to further conserve power when possible. In general, many control parameters of an implantable cardiac stimulation device can be adjusted based on rest detection to achieve various advantages.
Accordingly, aspects of the invention are directed to providing improved techniques for exploiting the determination of whether a patients is at rest.
Within some patients, arrhythmias may be vagally-mediated, i.e. decreased vagal tone can trigger the arrhythmia. Vagal tone relates to a basal level of activity in the body maintained by the vagus nerve. The vagus nerve, which is a portion of the parasympathetic nervous system, regulates the function of various organs and tissues including the heart by sending neural signals to the organs through efferent vagal fibers. The neural signals provided by the vagus nerve serve to maintain the basal level of activity within the body. Vagal tone is automatically increased or decreased by the parasympathetic nervous system in response to internal or external sensory stimuli depending upon the needs of the body. For example, when the patient is asleep, the parasympathetic nervous system decreases the vagal tone of the body, making various organs less active and, in particular, lowering the heart rate. The lower heart rate can result in PACs or PVCs, which, in turn, can trigger tachyarrhythmias, including VF. As a result, some patients prone to vagally-mediated arrhythmias are much more likely to suffer SCD while asleep, then while awake.
Thus, for patients prone to vagally-mediated arrhythmias, any lowering of the base rate during sleep during sleep can significantly increase the risk of arrhythmias, such as bradycardias and tachycardias, occurring during sleep. However, heretofore, pacemakers and ICDs have been programmed to administer therapy without regard to whether the patient is prone to vagally-mediated arrhythmias. Hence, the devices do not compensate for the higher risk within patients prone to vagally-mediated arrhythmias while the patients are asleep. Indeed, as noted, many pacemakers and ICDs actually lower the base rate while the patient is asleep and thereby possibly increase the risk of an arrhythmia within the patient. For patients prone to vagally-mediated arrhythmias, it may be preferable to instead increase the base rate while the patient is asleep. It may also be preferable to increase an “aggressiveness” of overdrive pacing while the patient is asleep so as to achieve a higher degree of overdrive pacing while the patient is at greater risk of a tachycardia.
Accordingly, aspects of the invention are also directed to providing improved techniques for controlling implantable cardiac stimulation devices for use in patients prone to vagally-mediated arrhythmias. In particular, aspects are directed to an improved control technique for use with patients prone to vagally-mediated arrhythmias that takes into account whether the patient is at rest and adjusts pacing parameters accordingly.